It really depends on the location of the warehouse and if it's close to a large urban area. Typically if it's close to a big urban area with good road networks you will have a higher demand and let the units more quickly.
In commercial property at the moment, and particularly the industrial market, occupiers still prefer to purchase rather than let. So you will find that if you were to sell a small unit (1k - 2k sq. ft.) then it would shift quickly enough, but letting it would mean that it could be on the market for longer. Commercial investment properties tend to be let on a FRI (full repairing and insuring) lease, so once it's let then there is very little legwork to do. If you're that interested in an industrial investment, would you not purchase a specific property that is geared towards this type of investment, already let with an established tenant with rent agreed, and a set lease term? Most of the larger agents would definitely have stuff like this on their books right now.
Most of the larger commercial estate agents do extensive research in the industrial market - I know that Savills (HOK), Jones Lang LaSalle, CB Richard Ellis and DTZ all make them available on their websites. A 7K sq. m. facility is a huge undertaking and investment and I would advise you to seek out one of the specialist teams in Ireland to get some input on what would work in the area and the specific market in the area. It's a niche market and you would want to retain somebody with a proven track record to give you an idea of what size and specification the new re-development of the single unit should be.
I know that there's a focus on industrial property in the Sunday Tribune this week and I would recommend that you go out and get a copy to see who's marketing what, in what area etc.